This invention relates to an apparatus for monitoring the presence of metallic foreign bodies in a textile lap which is fed, for example, into a carding machine.
According to a known method, the driving torque for the feed roller of the carding machine is limited by means of, for example, a slip clutch. The foreign body carried by the advancing lap enters the bite of the feed roller and increases the torque resisting the roller drive. By virtue of the slip clutch, the roller drive continues its rotation, while the feed roller is at a standstill. This known method, however, is adapted to react only to relatively large and hard foreign bodies (that is, impurities having a relative high physical resistance). Further, an arrangement achieving torque limitation involves substantial expense.
French Pat. No. 1,411,766 discloses another type of sensing system for stopping the feed rollers upon detecting foreign bodies in a textile lap. As disclosed in this patent, in case the textile lap, passed between the feed roll and the feed table, contains undesirable foreign bodies, the latter displace the feed roll vertically upwardly against a spring-biased contact which, in turn, electronically causes stoppage of the feed roller. The sensing proper of the foreign bodies is thus effected by mechanical contacting. Such a mechanical detection is again effective only in case of relatively large and hard foreign bodies. Further, the shiftable bearing for the feed roller is a likely source of malfunctioning. Also according to this French patent, apart from the mechanical detection of foreign bodies, the fleece delivered by the carding machine is optically monitored by means of a light beam and a light barrier to detect ruptures or other defects in the fleece. It is apparent that while light beam testing of various quality-determining properties of the fleece is feasible, a light-beam monitoring is not adapted for the detection of foreign bodies embedded in the textile material.
German Pat. No. 1,510,191 and British Pat. No. 1,147,374 disclose the application of a high-frequency electromagnetic field across the running textile lap for the purpose of directly eliminating metallic foreign bodies. As the foreign body passes through the high-frequency field, it is heated thereby and thus it burns its way through the textile lap and is in this manner directly eliminated therefrom. It is a disadvantage of this method that it involves a local weakening and destruction of the lap. Thus, the method disclosed in these two patents is concerned with the direct elimination of foreign bodies and gives thus no consideration to a detection that makes possible a subsequent gentle removal of foreign bodies.